Holography is an optical phenomenon for which a rising interest in practical applications has occurred since about 1960. Briefly, holography is an optical process by which an object is illuminated by a pulsed collimated beam of light such as from a pulse type laser and the reflected light therefrom (called an object beam) is impinged along with a reference beam of light onto a holographic element or recording medium which is then developed and viewed through to provide a holographic image of the object. The holograhic element may be a high-resolution photographic film, a photoresist, or gelatin impregnated with light sensitive ammonium dichromate as is well known to those skilled in the art of holography. The interference and object beams interfere to create an extremely fine pattern of light and dark fringes on the surface of the holographic element which when developed enable the developed to provide an image identical to the original object when illuminated with a replica of the reference beam. Generally, holograms are similar to refraction gratings which deflect light of different wave lengths at different angles with the added advantage of being able to reproduce an image of the original object as previously described.
The holographic process provides a means of "fingerprinting" an image of the original object by means of refractance interference lines which characterize the object and which change when a physical abberation is present or occurs in the object between successive exposures to the holographic element. Thus, optical intereference refractance lines appearing in the holographic image can be used to determine the nature and extent of physical changes and/or stresses occuring in an object subjected to the holographic process over spaced periods of time.
Examples of practical applications of the holographic process are respectively disclosed in U.S. Pat. No. 3,509,761; for determining deformation stress in an object, and in U.S. Pat. No. 3,612,693; for determining corrosive effects on the surface of an object, and in U.S. Pat. No. 3,709,605; for determining dynamic variations occurring in a singular rotating object, the disclosures of all of which are incorporated herein by reference.
An area of concern for many years has been to provide a simple and inexpensive method of determining changes occurring in gear sets, particularly gear sets such as a differential ring or pinion gears that engage each other at 90 degrees, under dynamic rotating conditions. Under rotating conditions shape changes and vibrating displacement conditions may occur which are virtually undetectable by ordinary measurement techniques including conventional high speed photography and stroboscopic principles. It has been discovered that the holographic process is able to satisfy this concern by providing a simple, inexpensive, and highly accurate method of determining dynamic rotational characteristics of gear sets having mutual orthogonal rotational axis.